OFDM (Orthogonal Frequency-Division Multiplexing) is becoming widely used in wireless communication systems due to its efficient implementation and its robustness with regard to multi-path fading. Recently, OFDM has been adopted as a primary modulation technique for some mobile TV systems, such as Korean terrestrial digital multimedia broadcasting (T-DMB), European digital video broadcasting for handheld devices (DVB-H), and MediaFLO mobile-TV system deployed in United States.
One problem in mobile TV applications is mobile signal reception in, such as cars, buses, high-speed trains and so on. In such situations, the receiver should accurately estimate time-varying channel response. This is typically accomplished by adding to the OFDM signal a predefined number of scattered pilot cells, which are uniformly distributed in the time and frequency domains.
However, even though the channel estimation may be accurate, the performance of the OFDM receiver in time-varying channels can be limited by the ICI effect introduced by channel response variation during the duration of one OFDM symbol. One approach to this problem is to use an OFDM system with a shorter OFDM symbol duration to improve mobile performance. Another approach is to use dedicated ICI cancellation circuits or a special form of the channel equalizers that take into account the time-varying effect of the channel within the duration of one OFDM symbol, as discussed, for example, in references [1] and [3]-[7] listed herein.
FIG. 1 is a functional block diagram showing the structure of a conventional OFDM receiver using a special form of the channel equalizers. Referring to FIG. 1, in a conventional OFDM receiver 10, a signal that is up-converted by an OFDM transmitter (not shown) is down-converted by a down-converter (not shown) and an analog-to-digital converter (not shown) converts the down-converted signal to a digital signal and outputs the digital signal to a GI (guard internal) remover 11. The GI remover 11 removes from the input digital signal a guard interval inserted by the OFDM transmitter to estimate channels. The signal output from the GI remover 11 is converted to a frequency domain signal by an FFT (Fast Fourier Transform) portion 12. The ICI of the signal converted to the frequency domain can be canceled or compensated for by an equalizer 13. The OFDM receiver 10 may further include a decoder (not shown) to decode a signal output from the equalizer 13.
FIG. 2 shows a GI insertion scheme. Referring to FIG. 2, predetermined data at the rear portion of the original OFDM symbol is copied to be used in the GI. Thus, the OFDM symbol consists of the GI and an active OFDM symbol that is referred to as an effective OFDM symbol. Typically, GI(TGI) is about ¼, ⅛, or 1/16 of an active OFDM symbol section (TU).
The OFDM transmitter generates the OFDM symbol of FIG. 2 and continuously transmits the generated symbol. Then the OFDM receiver as shown in FIG. 1 continuously receives the OFDM symbol of FIG. 2 to process each OFDM symbol. The OFDM receiver discards the GI from each OFDM symbol.
When a multi-path delay-spread is shorter than the GI, the OFDM system may be able to handle the multi-path echo. Thus, the GI can be selected as a sufficiently long section to reduce degradation of performance in a channel with a long echo. A very long echo can be intentionally generated in a network referred to as a single frequency network (SFN). In the SFN, the same information is broadcasted by many transmitters using the same frequency to increase the effective coverage domain without increasing the power of a transmitter. In this case, a user can receive two signals having almost the same size but different delays.
Therefore, a system designed for an SFN operation may handle a worst case delay time difference between two arrival paths. For example, a DVB-T SFN network may exceed the worst case delay time of 200 μs. Thus, a long GI may be used to reduce the performance degradation in the case worst scenario. In particular, many systems use a GI that is ¼ of the active OFDM symbol to reduce the performance degradation in the case worst scenario.
In a normal environment, the multi-path delay-spread may be much shorter than the worst case scenario. For example, the multi-path delay may be generally shorter than 10 μs in digital TV channels. Thus, in a normal channel environment, the OFDM system designed for the worst case situation may actually cause loss in transmission speed or signal-to-noise ratio (SNR) due to the transmission of a long extra cyclic prefix that is disregarded in the receiver.
References referred to herein include:
[1] P. Schniter, “Low-complexity Equalization of OFDM in Doubly Selective Channels,” IEEE Trans. Signal Processing, Vol. 52, No.4, April, 2004, pp.1002-1011
[2] Y. Mostofi, D. C. Cox, “ICI Mitigation for Pilot-Aided OFDM Mobile Systems,” IEEE Trans. Communications, Vol.4, No. 2, March, 2005
[3] Jean-Paul Linnartz, A. Filippi, S. A. Husen, S. Baggen, “Mobile reception of DVB-H: how to make it work?”, 3rd IEEE BENELUX/DSP Valley Signal Processing Symposium SPS-DARTS 2007, Antwerp, Mar. 21-22, 2007
[4] S. A. Husen, S. Baggen, M. Stassen, H. Y. Tsang, “Simple Doppler compensation for DVB-T”, 25-th Symposium on Information Theory in the Benelux, The Netherlands, Jun. 2-4, 2004
[5] Luca Rugini, Paolo Banelli, and Geert Leus, “Simple Equalization of Time-Varying Channels for OFDM,” IEEE COMMUNICATIONS LETTERS, VOL. 9, NO. 7, July 2005
[6] C. Oria, et al., “Optimum Doppler compensation scheme for DVB-H receivers,” ELECTRONICS LETTERS, 22 Jun. 2006 Vol. 42 No. 13
[7] K. Schmidt, C. Gunter, A. Rothermel, “Improving the mobility of dvb handheld devices with inter-carrier interference compensation,” Consumer Electronics, 2004 IEEE International Symposium on Sep. 1-3, 2004